Huanglongbing (also known as HLB, yellow shoot/yellow dragon disease and citrus greening) is the most destructive disease ever known to the citrus world. Oranges have been the most adversely affected cultivar though grapefruit, tangerine, lime and some lemon have fallen prey to this killer. Greening is caused by the bacterium Candidatus Liberibacter, which chokes the vascular system of trees preventing the phloem (the living tissue responsible for transporting nutrients throughout trees) from allowing nutrients to properly reach the limbs, leaves, trunk and roots of the infected trees.
An early symptom, which is difficult to recognize as it mimics some nutrient deficiencies, is yellowing of the leaves where one side of the leaf turns yellow and the other side has blotchy patches of green. Longer terms symptoms include reduced tree height, stunted flush and fruit that is small, green, lopsided and fails to ripen. The disease also dramatically increases pre-harvest fruit drop and produces abnormally bitter fruit with a high acid content. Generally, within five to eight years infected trees will die, destroying entire citrus groves. This is a real threat for the entire citrus industry.
Which Areas of the World are Affected?
HLB has been reported in 40 countries across the globe. In China, where greening originated, it was first identified and reported in 1919. Candidatus Liberibacter asiaticus is the strain that widely affects Asia and North America where it thrives in the warm climates. The disease was first found in Florida in 2005 and unfortunately spread to every commercial grove across the state in less than three years. Other strains include Candidatus Liberibacter americanus in Brazil and Candidatus Liberibacter africanus in Africa, which prefers cooler climates.
How Does the Disease Spread?
The disease is spread by the Asian citrus psyllid (Diaphorina citri Kuwayama), a gnat-sized insect who feeds on the leaves and stems of citrus trees. When the bugs feed on the trees, they inject the Liberibacter bacteria into those trees, causing greening. Adult psyllids then fly or leap from tree to tree and grove to grove infecting as they go. Hurricanes always bring with them increased spread of the psyllids as they are tiny (average 0.125 inches in width, 3mm in length) and can travel easily through wind. The transport of infested rootstalks and trees can also spread HLB, so greenhouses today have numerous controls to prevent this from happening.
Preventative Management Until a Cure is Developed
There is no proven cure for HLB and no resistant citrus varieties currently available to withstand the disease. Hundreds of rootstalk/budwood combinations are being grafted and tested for improved resistance. Extensive research into cures is being conducted in laboratories all over the world. Unfortunately for research efforts Liberibacter pathogens cannot be cultured in the lab which limits the understanding of the infection mechanisms and delays development of potential treatments.
Currently, eradication of the disease is unlikely, but management strategies are in place to slow the spread of the disease and increase production in the groves. These include removing infected trees immediately, planting new disease-free stocks, high-density citrus planting, psyllid management and promoting good root health. Keeping trees well-nourished has been one of the industry’s best ways to fight back. This certainly doesn’t stop greening but, by keeping the trees as healthy as possible, it takes longer for the disease to debilitate the tree and affect the fruit.
Brazil has been much more successful at keeping the disease at bay because of their expediency in removing diseased trees and replanting healthy ones. Only between 17-23% of citrus trees there have the disease; whereas about 80% of all Florida citrus are infected. California has been quite successful at keeping citrus groves healthy. The psyllid was not even seen in California until 2008 and the first case of an infected tree was identified only in 2015. So, farmers are at least learning to live with the disease and are invested in maintaining the best practices until a cure is discovered.
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